The present invention relates to data storage. More particularly, the present invention relates to a digital storage media library having failover capability.
Digital data storage devices are utilized for storing information for use by data processing systems including computer systems. One commonly used data storage medium is tape storage, used in tape libraries, well suited for backup operations as well as for providing archival and retrieval operations for vast quantities of information content. In this regard, optical storage is also known for voluminous content storage and retrieval.
Tape libraries are known in the art. One example of a tape library is provided by the Ostwald U.S. Pat. No. 5,236,296. In that patent, a tape library is described in FIG. 8 as comprising a vast, semi-cylindrical array of tape cartridge storage slots aligned generally along a fixed radius of curvature. A central cartridge inventory is maintained by a library controller, so that logical requests for a particular drive and cartridge may be translated by the library controller into physical device locations and electromechanical operations. In this prior example, a media loader includes a robotic arm rotating at a focus of the cylindrical segment that is elevated and rotated to a particular cartridge storage slot. A picker-gripper mechanism of the arm then xe2x80x9cpicksxe2x80x9d and xe2x80x9cgripsxe2x80x9d the cartridge stored in the slot and moves the cartridge out of the slot and into a temporary transport slot of the arm. The robotic arm is then commanded to perform a second rotation/elevation operation in order to present the retrieved tape cartridge to a loading tray of the selected tape drive, and the drive then loads the cartridge and threads the tape for recording/playback operations, following initial setup and calibration routines conventional with tape drives. The drive may be one of several drives accessible by the robotic arm.
Typically, media loaders (e.g. tape cartridge loader) operate in accordance with a standardized command structure. One such command structure is found in the Small Computer System Interface-2 draft standard X3T9.2 Project 375D (ANSI X3.131-199X). In this particular industry specification, a medium changer device includes a medium transport element, at least one storage element, and a data transfer element. An import/export element may also be supported. A storage element is identified as a storage slot for storing a standard medium unit, such as a disk or a tape cartridge. Typically, in order to access data on a standard medium unit, a host system will have to issue commands to both the medium loader and to the drive. The commands to the loader may include xe2x80x9cmove mediumxe2x80x9d; or, xe2x80x9cexchange mediumxe2x80x9d and xe2x80x9cread element statusxe2x80x9d. Commands directed by the host to the drive may include xe2x80x9ctest unit readyxe2x80x9d, xe2x80x9cinquiryxe2x80x9d, xe2x80x9cstart-stopxe2x80x9d and xe2x80x9cload-unloadxe2x80x9d commands, in addition to the obvious xe2x80x9cread/writexe2x80x9d commands. One important characteristic about this command structure is that the logical address of the drive is supplied to the media loader as a destination, as well as to the drive itself for subsequent read or write operations from or to the selected and automatically loaded medium unit.
Storage devices such as tape libraries are required to provide fail safe mechanisms for availability and reliability. Tape drives are complex electromechanical devices subject to many forms of failures including those which occur outside of normal data transfer operations. For example, ingestion of the tape leader by a tape drive causes the tape drive to fail and become inoperative. As such, host commands for transferring data to/from tape cartridges via a tape drive that has failed remain unserviced, negatively affecting the operation of host tape access. Conventional tape libraries do not address tape drive failures that occur outside of normal data transfer operations. There is, therefore, a unsolved need has remained for a reliable and fail safe architecture for a digital storage media library for overcoming significant limitations and drawbacks associated with the conventional media libraries.
The present invention satisfies these needs. In one embodiment, the present invention provides a process/method for controlling a digital data storage unit. The digital data storage unit includes a multiplicity of storage media slots for receiving media storage units, a plurality of media storage units loaded in some of the storage media slots, a plurality of data storage drives each having a unique drive address, a loader mechanism for selectively moving a media storage unit between a storage media slot and one of the plurality of data storage drives, and a storage unit controller connected to at least one host computer.
The process for controlling the digital data storage unit is executed by the storage unit controller. Initially, one or more of the data storage drives are reserved as spare data storage drives, wherein the spare data storage drives are masked from the host computer such the spare data storage drives are not directly accessible by the host. The storage unit controller then receives and decodes host commands including a source address corresponding to a storage media slot location, and a destination address corresponding to a data storage drive specified by the host computer.
The controller determines if the specified data storage drive is available. If the specified data storage drive is not available, the controller performs xe2x80x98failoverxe2x80x99 processing by using a spare data storage drive in place of the specified data storage drive. In one version, the controller redirects the host commands from the specified data storage drive to the spare data storage drive. In another version, the controller redirects data flow between the host and the specified data storage drive to the spare data storage drive. The controller further, controls the loader to move the media storage unit at the storage media slot location to the spare data storage drive. As such, the present invention provides a reliable and fail safe architecture for a digital storage media library that overcomes significant limitations and drawbacks associated with the conventional media libraries.